1. Field of the Invention
The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same, and more particularly, to an electrolyte for a lithium secondary battery that may prevent the battery from swelling while maintaining its electrochemical properties, and a lithium secondary battery comprising the same.
2. Discussion of the Related Art
Due to recent trends toward more compact and lighter portable electronic equipment, there has been a growing need to develop high performance and large capacity batteries. Lithium secondary batteries with good safety characteristics and improved electrochemical properties have been extensively researched. These batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions for positive and negative active materials. The positive active materials may include lithium metal oxide and the negative active materials may include lithium metals, lithium-containing alloys, or materials that are capable of reversible intercalation/deintercalation of lithium ions such as crystalline or amorphous carbons, or carbon-containing composites.
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7V, which is higher than alkali batteries, Ni-MH batteries, Ni—Cd batteries, and other similar batteries. An electrochemically stable electrolyte in the charge and discharge voltage range of 0 to 4.2V is required in order to generate such a high driving voltage. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate is typically used as an electrolyte. However, this electrolyte may have significantly lower ion conductivity than an aqueous electrolyte used in a Ni-MH or Ni—Cd battery, thereby resulting in battery characteristic deterioration during high rate charging and discharging.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the battery's lithium-transition metal oxide positive electrode, are transferred to a carbon negative electrode, where they are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, and the like, thereby forming a thin organic solid electrolyte interface (SEI) film on the negative electrode's surface. The initially formed organic SEI film prevents a reaction between lithium ions and the carbon negative electrode, or other materials during charging and discharging. It also acts as an ion tunnel that allows the passage of only lithium ions and prevents the carbon negative electrode's disintegration, which is caused by co-intercalation of organic solvents having a high molecular weight along with solvated lithium ions into the carbon negative electrode.
Once the organic SEI film is formed, lithium ion numbers are not reduced because they do not react again with the carbon electrode or other materials. In other words, stable charging and discharging are maintained because when the carbon of the negative electrode initially reacts with an electrolyte, a passivation layer, such as an organic SEI film, is formed on the negative electrode's surface and prevents the electrolyte solution from decomposing (Journal of Power Sources, 51(1994), 79-104). For these reasons, in the lithium secondary battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle-life may be maintained after initial charging.
However, during the organic SEI film-forming reaction, gases are generated inside the battery due to decomposition of a carbonate-based organic solvent (J. Power Sources, 72(1998), 66-70). Depending on the type of non-aqueous organic solvent and negative active material used, these gases may include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, and the like. Internally generated gases may increase battery thickness.
Electrochemical and heat energy slowly disintegrate the passivation layer. Disintegration may increase over time when the battery is stored at a high temperature after charging. As a result, a continuous side reaction occurs in which an exposed surface of the negative electrode reacts with surrounding electrolyte to generate gases, which increases the battery's internal pressure and induces deformation of prismatic or pouch battery. Consequently, regional differences in the cohesion among electrodes inside the electrode assembly (positive and negative electrodes, and separator) of the battery occur, thereby deteriorating the battery's performance and safety, which makes it difficult to mount the lithium secondary battery set into electronic equipment.
In order to improve low temperature characteristics, a lithium secondary battery having a liquid electrolyte uses an organic solvent with a low boiling point. But a prismatic or pouch battery may swell during high temperature storage, which deteriorates the battery's reliability and safety at a high temperature.
Accordingly, extensive research into a liquid electrolyte with a high boiling point is needed. An ester solvent, such as gamma butyrolactone, is an example of an electrolyte with a high boiling point. When using 30 to 70% of an ester solvent, however, cycle-life characteristics may significantly deteriorate. In order to reduce swelling at a high temperature and improve cycle-life characteristics, an electrolyte with a high boiling point, a mixture of gamma butyrolactone/ethylene carbonate (7/3), and a boron-coated mesocarbon fiber (MCF) as a negative active material have been suggested (Journal of Electrochemical Society, 149(1) A(9)˜A12(2002)). However, when an uncoated carbonaceous material is used as a negative active material, cycle-life characteristics may deteriorate even when an electrolyte with a high boiling point is used.
U.S. Pat. Nos. 5,352,458 and 5,626,981 disclose an electrolyte comprising vinylene carbonate in order to overcome cycle-life deterioration shortcomings of a high viscosity electrolyte. However, sufficient cycle-life improvement may not be obtained.
U.S. Pat. No. 5,529,859 discloses a battery with improved performance that uses an electrolyte prepared by adding a halogenated organic solvent such as chloroethylene carbonate to propylene carbonate. U.S. Pat. No. 5,571,635 discloses a battery with improved performance that uses an electrolyte prepared by adding a halogenated organic solvent such as chloroethylene carbonate to a mixed solvent of propylene carbonate and ethylene carbonate. However, the propylene carbonate has a high viscosity, and it may decompose when it is intercalated into a carbon layer of crystalline negative active material such as graphite. Decomposition products of propylene gas and lithium carbonate may result in deterioration of capacity and increase of irreversible capacity. In the above patents, propylene carbonate and chloroethylene carbonate is mixed in a volume ratio of 1:1, which may deteriorate electrolyte wettability.